1. Field of the Invention
The invention relates to an improved method of and an apparatus for electroerosive material machining of a workpiece with an elongated tool electrode.
2. Description of the Prior Art
In electroerosive material machining (this term here refers to a machining method that provides for the removal of material from a conductive workpiece, at least in part by the action of electrical discharges), a tool electrode is placed spaced apart from and opposite a workpiece across a work gap. The work gap is flushed with a work fluid, and an erosive machining current, conventionally in the form of a train of electrical pulses, flows between the tool electrode and the workpiece. Successive electrical discharges are thus generated through the fluid-flushed work gap, in order to remove material from the workpiece. Depending on the progress of the removal of material, the tool electrode held by a tool head is advanced by a control system, in order to keep the work gap, which would otherwise increase in size, essentially constant.
In the erosion process, an erosive wear of the machining face of the tool electrode can occur. Moreover, after every discharge, a change in the conditions in the work gap occurs, caused by a change in the properties of the dielectric, such as soiling, temperature and so forth, which for instance change the electrical conductivity. Not only the electrical parameters but the erosion process is affected by the flushing. In bores with a large aspect ratio, it often happens that the removal process fails, since the products of removal can no longer be removed from the gap. As a consequence, besides normal spark discharges, so-called degenerations occur, which markedly affect the removal and wear. Idle pulses (the work gap is too large), faulty discharges (the work gap is too small), and short-circuit pulses (the work gap equals zero), in particular, come under the category of degeneration.
If a good erosion process is to be assured, ideal discharge conditions in the work gap must be created, which as much as possible preclude the occurrence of short circuits, faulty discharges and idle pulses. The essential criterion here is for interference variables to be detected by means of suitable process parameters for assessing the instantaneous status of the process. Various strategies, which may include various target functions, then lead to suitable tracking of the controlling variables in the machine.
U.S. Pat. No. 4,771,157, discloses a process in which the erosion process is monitored by a short-circuit detector, and in accordance with the input data, the tool electrode is moved in the axial motion via a control system, to make it possible to keep the work gap essentially constant.
Recently, the use of fuzzy logic, by means of the formulation of automatic control mechanisms, makes it possible to combine various parameters into a multi-variable automatic control system. Thus the machine parameters are even better-adapted to the current state of the process and lead to the desired outcome of the work.
However, it is very important that a control system put the tool electrode into the desired position quickly and accurately in accordance with the process state. Faster and faster and above all more and more expensive control systems are therefore used. To reduce the inertia of the control system, the attempt is made to reduce the mass of the moving part of the control system still further. For this purpose, some apparatuses have two control systems: one control system that puts the tool electrode coarsely in the vicinity of the workpiece, and the more-accurate and above all faster control system which as the process continues determines the fine setting of the work gap. A further problem is so-called microspark erosion, in which the tool electrode is very thin and has a diameter between 20 μm and 200 μm. Until now, for microscopic bores smaller than 0.1 mm and an aspect ratio between the diameter and depth of 1:10, microspark erosion has not established itself in large-scale mass production. However, with such dimensions, electrode manipulation also proves to be problematic.
The object of the invention is to refine a method and an apparatus of the type defined at the outset, by which a direct, fast and simple control system for regulating the work gap is to be created, and the tool electrode itself is an active component of the control system. A further object of the invention is simple electrode manipulation; that is, picking up, aligning and chucking of the tool electrode for continuous machining of the workpiece to form a desired opening should be designed such that usage in large-scale mass production is made possible. The term “opening” here means in particular, but not exclusively, so-called microscopic bores in which the electrode diameter is between 20 μm and 200 μm.